1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus that magnetically excites an atomic nuclear spin of an object by using a radio frequency (RF) signal having a Larmor frequency and reconstructs an image from a nuclear magnetic resonance (NMR) signal produced with the excitation and to an RF coil system for use therein. In particular, the present invention relates to an MRI apparatus that wirelessly transmits a received signal, an RF coil system for use therein, and an MRI method.
2. Description of the Related Art
MRI is an imaging method of magnetically exciting an atomic nuclear spin of an object placed in a static magnetic field by using an RF signal having a Larmor frequency and reconstructing an image from an NMR signal produced with the excitation.
The number of RF coils for receiving an NMR signal in recent MRI apparatuses has a tendency to be increased. In known MRI apparatuses, NMR signals are received using all RF coils, and an image is generated by the received NMR signals. As a result, an MRI apparatus has reception channels of the same number as the RF coils and generates the image by data processing of a reception signal corresponding to each of the reception channels.
However, imaging needs data related to processing the reception channels, so the length of time required for the processing can be long. An increase in the processing time results in an increase in time required for obtaining the image of a patient as the object, thus putting a heavy load on the patient.
One known technique for storing attribute information in RF coils to locally collect necessary data using only the RF coils is disclosed (see, for example, Japanese Patent Application Publication No. 2001-346775). In this technique, the RF coils can be identified based on the attribute information, and necessary data can be selectively collected using a specific RF coil. This reduces the amount of data processing and the length of processing time.
RF coils used in a known MRI apparatus are electrically connected to a controller with a conductive cable, and reception signals output from the RF coils and control signals are transmitted via the cables. However, the cable not only hinders the placement of the patient on a table-top of a bed system but also incurs the risk of burning the patient by RF induction caused when the patient comes into contact with the cable.
One approach to this problem is a technique for transmitting a signal without electrically connecting an RF coil to a controller. One example of the technique is to convert a signal output from the RF coil from electrical to optical form. For this technique, although it is necessary to connect the RF coil to an optical cable, the optical cable used in signal transmission can be relatively thin. The use of an optical cable in signal transmission can reduce the risk of producing a burn because there is no electrical contact with a body of the patient.
Another example of means of transmitting a signal using non-electrical connection is a technique for converting a signal into an electromagnetic wave and performing wireless transmission from an RF coil to a controller. That is, a technique for converting an electrical signal output from an RF coil used for signal reception in obtaining an image into a radio wave and transmitting it to the controller is discussed.
However, when the electrical signal output from the RF coil is transmitted as a radio wave to the controller, it is difficult for an MRI apparatus that can position a plurality of RF coils in the vicinity of the patient to automatically identify an RF coil positioned for collecting data at the controller.
For example, a plurality of RF coils typically exists in an imaging room where the MRI apparatus is set. During imaging, an RF coil corresponding to an imaging target section is positioned above or adjacent to the patient. However, RF coils that are not positioned above or not adjacent to the patient also receive signals. Signals from such RF coils are unnecessary signals from sections other than the imaging target section of the patient. It is thus preferred that the signals from the RF coils that are not positioned above nor adjacent to the object be removed. However, the controller cannot identify from which RF coil the collected data has come, so it is difficult to remove the unnecessary data.
Because of this, it is necessary to wirelessly transmit signals received in all RF coils to the controller and extract only data required for diagnosis after the completion of data processing. However, this leads to an increase in unnecessary data processing.